write part 16

[rust-101.git] / src / part04.rs

diff --git a/src/part04.rs b/src/part04.rs
index f39df37148f89efd586bc0af778226272929ae0e..5c894669462c9ac95f4f145ddd0e887f4aab5ab0 100644 (file)
--- a/src/part04.rs
+++ b/src/part04.rs
@@ -27,7 +27,7 @@ fn work_on_vector(v: Vec<i32>) { /* do something */ }
 fn ownership_demo() {
     let v = vec![1,2,3,4];
     work_on_vector(v);
-    /* println!("The first element is: {}", v[0]); */
+    /* println!("The first element is: {}", v[0]); */               /* BAD! */
 }
 //@ Rust attaches additional meaning to the argument of `work_on_vector`: The function can assume
 //@ that it entirely *owns* `v`, and hence can do anything with it. When `work_on_vector` ends,
@@ -68,7 +68,9 @@ fn vec_min(v: &Vec<i32>) -> Option<i32> {
     use std::cmp;
 
     let mut min = None;
-    for e in v {
+    // This time, we explicitly request an iterator for the vector `v`. The method `iter` borrows the vector
+    // it works on, and provides shared borrows of the elements.
+    for e in v.iter() {
         // In the loop, `e` now has type `&i32`, so we have to dereference it to obtain an `i32`.
         min = Some(match min {
             None => *e,
@@ -102,9 +104,9 @@ fn shared_borrow_demo() {
 
 //@ As an example, consider a function which increments every element of a vector by 1.
 //@ The type `&mut Vec<i32>` is the type of mutable borrows of `vec<i32>`. Because the borrow is
-//@ mutable, we can change `e` in the loop.
+//@ mutable, we can use a mutable iterator, providing a mutable borrow of the elements.
 fn vec_inc(v: &mut Vec<i32>) {
-    for e in v {
+    for e in v.iter_mut() {
         *e += 1;
     }
 }
@@ -114,10 +116,12 @@ fn mutable_borrow_demo() {
     /* let first = &v[0]; */
     vec_inc(&mut v);
     vec_inc(&mut v);
-    /* println!("The first element is: {}", *first); */
+    /* println!("The first element is: {}", *first); */             /* BAD! */
 }
 //@ `&mut` is the operator to create a mutable borrow. We have to mark `v` as mutable in order to create such a
-//@ borrow. Because the borrow passed to `vec_inc` only lasts as long as the function call, we can still call
+//@ borrow: Even though we completely own `v`, Rust tries to protect us from accidentally mutating things.
+//@ Hence owned variables that you intend to mutate, have to be annotated with `mut`.
+//@ Because the borrow passed to `vec_inc` only lasts as long as the function call, we can still call
 //@ `vec_inc` on the same vector twice: The durations of the two borrows do not overlap, so we never have more
 //@ than one mutable borrow. However, we can *not* create a shared borrow that spans a call to `vec_inc`. Just try
 //@ enabling the commented-out lines, and watch Rust complain. This is because `vec_inc` could mutate